An in-line type electron gun is constituted such that its cathode as a source of electron beams are positioned on a common plane, and a converging means is provided to converge the electron beams generated from the cathodes into a small spot on the screen of cathode ray tube. Such a converging means is provided in a main lens means which comprises a single main focus lens. Such a converging means is constituted as schematically illustrated in FIGS. 1, 2 and 3, and explained in U.S. Pat. No. 3,772,554.
As shown in FIGS. 1 and 2, such a single lens type electron gun comprises: cathodes K for emitting thermal electrons; a control grid G1 and a screen grid G2 for transforming emitted thermal electrons into electron beams; and a focus electrode G3 for focusing transformed electron beams and an anode G4 for accelerating and refocusing focused electron beams. In such an electron gun, the converging means is constituted such that outer beam passing holes 4Ha' of anode G4 are positioned outwardly enlarged relative to beam outgoing outer beam passing holes 3Ha of focus electrode G3.
Since such a converging means has upstream and downstream beam passing holes eccentrically aligned with respect to one another, there are several limitations in designing such a converging means.
As shown in FIG. 3, the diameters of outer beam passing holes 4Ha' of anode G4 are designed to be greater than those of outer beam passing holes 3Ha of focus electrode G3. However, looking from the side of the screen of cathode ray tube opposite cathodes K, outer beam passing holes 3Ha should not be hidden behind, but should be exposed in its entirety inside outer beam passing holes 4Ha'. If outer beam passing holes 3Ha of focus electrode G3 are hidden behind outer beam passing holes 4Ha' of anode G4, a straight guide rod (not shown) cannot be inserted.
Therefore, generally, as shown in FIG. 3, the innermost circumference of outer beam passing holes 4Ha' is made to be tangent with that of outer beam passing holes 3Ha. Thus, the diameters of outer beam passing holes 3Ha should be smaller than those of outer beam passing holes 4Ha'.
However, if outer beam passing holes 4Ha' are enlarged for ease of assembly of the electrodes, then there arise discrepancies between the electric fields of outer beam passing holes 3Ha and 4Ha' and middle beam passing holes 3Hb and 4Hb, resulting in differences in characteristics of the R, G, B beam between the middle electron beam and outer electron beams, which in turn result in degradations of picture quality.